Once again, dark matter has failed to turn up where researchers hoped they might find it.
PandaX — essentially a tank holding 1,280 lbs. (580 kilograms) of liquid xenon beneath the Jinping Mountains of Sichuan, China — is one of the most sensitive dark-matter detectors on the planet. If dark matter is capable of bumping into the matter we can detect, and if dark matter is made up of big, bulky particles called WIMPs (weakly interacting massive particles) as scientists have long assumed, then sooner or later, some of the dark stuff should knock into xenon particles inside PandaX in a way researchers can detect.
But recently reported data from an 80-day experiment at the facility, which was completed in 2015, tells physicists that hasn't happened. And that null result, the umpteenth null result in the hunt for dark matter, tells us something about dark matter.
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Dark matter is the big missing puzzle piece of the universe. When scientists study the way stars move through space and the large-scale structure of the universe, they can see that something's missing. Everything we can see behaves as if there's a lot more matter out there exerting a gravitational pull than shows up in our telescopes. That dark, missing matter accounts for at least 80 percent of the mass of the universe. But researchers have no idea what it's made of. [The Coolest Little Particles in Nature]
In a paper published July 12 in the journal Physical Review Letters, a team of researchers interpreted the null data from PandaX to put new limits on what dark matter could possibly be — and the work offers possible alternative explanations for what could really be out there.
The basic process of elimination reported in the paper seems pretty simple: Dark matter is unlikely to be made up of particles that interact meaningfully with ordinary matter and have masses between about 5 million and 10 million times the mass of a proton.
But that's a big deal, as Hai-Bo Yu, a physicist at the University of California, Riverside and co-author of the paper, explained.
It shows, he said, that certain proposed explanations for dark matter — most importantly, WIMPs, which should show up in an experiment on the scale of PandaX — are likely incorrect. Dark-matter particles are likely much smaller than WIMPs would have to be, he said, and may not behave in ways that make them easy to study.
"We have to be prepared for the idea that dark matter might not interact with other matter except through gravity," Yu told Live Science.
Based on the limitations placed on dark matter by PandaX and other experiments, Yu and his colleagues are moving toward the conclusion that the best laboratory for understanding dark matter might be the night sky. Stars and galaxies exhibit subtle behaviors that researchers can use to glean information about dark matter.
And astronomical observations, Yu said, point increasingly toward a model called self-interacting dark matter — particles that interact primarily with one another through unknown means, rather than interacting primarily (or interacting at all) with the ordinary matter we're used to. And the best way to observe dark matter of this sort, he said, is through its effects on what we can see in outer space.
(Not every scientist agrees, of course. There are even some skeptics who doubt that dark matter exists at all, though they're increasingly in the minority.)
Still, Yu said, there's room for experiments here on Earth to probe deeper for dark matter and, at the very least, perform more process of elimination. A bigger, heavier xenon chamber that's capable of looking for much smaller possible dark-matter particles, like hypothetical elementary particles called axions, would be a good place to start, he said.
Originally published on Live Science.
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